High temperature operating environments such as those present in gas turbine engines, power generation turbines, refinery equipment, and heat exchangers demand parts composed of a variety of cobalt-, iron-, and nickel-base metals known as superalloys. These superalloys are capable of withstanding extremely high temperatures for extended periods of time, but the extremely stressful temperature conditions to which superalloy articles are subjected eventually takes its toll upon the metal in a number of ways.
The main types of damage to a superalloy article are cracks from thermal fatigue, wide gap cracks, foreign object impact damage, and dimensional reduction from mechanical wear. Because the cost of these superalloy components is quite high, there is considerable incentive to repair these types of defects rather than to scrap the part and replace it with a new one. The high cost of these components, as well as the fact that superalloy components, once damaged, tend to fail repeatedly in the same region, also makes it critical that any repairs made have mechanical, environmental, and processing properties equivalent to or better than the original superalloy base metal.
Traditional methods for repairing damaged superalloy articles involve choosing or creating an alloyed combination of elements that will melt at a temperature below the melting temperature of the superalloy substrate. These compositions are known in the industry as braze alloys, and the most useful prior art braze alloys are characterized as either nickel-base or cobalt-base alloys. Historically, the most popular braze alloys contain a melting point depressant such as silicon or boron; a complex of some of the same alloying elements used in the superalloy article to be repaired such as chromium, aluminum, titanium, tungsten, etc.; and either nickel or cobalt as the base. In fact, one braze alloy, sometimes known as B-28, is simply the combination of an alloy frequently used to manufacture cast turbine airfoils, named Rene'80, with about 2% boron.
Advances in the braze alloy composition art have brought multi-constituent alloy compositions which are mixtures of at least one braze alloy and at least one base metal alloy, the base metal alloy differing from the braze alloy in that it melts at a higher temperature than the braze alloy and contains no melting point depressants which can weaken the repair site. These multi-constituent compositions result in stronger repairs because the low-melting brazing alloy liquefies first, wetting the base metal constituent and joining the entire mixture to the superalloy article.
Once a braze alloy or alloy mixture has been chosen, the damaged superalloy article is cleaned to remove any environmental coating that may be over the base metal and any oxides that may have developed inside the damaged regions. The braze alloy composition is then applied to the region to be repaired, and the article subjected to a high temperature brazing cycle to melt and join the braze alloy to the superalloy article. Upon the completion of this cycle, typical braze alloys will have formed undesirable large blocky or script-like brittle phases composed of chromium, titanium, and the family of refractory elements (e.g., tungsten, tantalum) combined with the melting point depressants. These brittle phases weaken the repair composite and cannot be removed from conventional braze alloys.
However, certain braze alloy compositions, known as diffusion braze alloys, are capable of withstanding higher temperatures than conventional braze alloys. Diffusion braze alloys form the same bad phases during brazing as conventional alloys, but diffusion braze alloys can be subjected to a second, long-term high temperature heat cycle known as a diffusion cycle. This diffusion cycle allows the brittle borides, carbides, and silicides to break down into fine, discrete blocky phases. The diffusion cycle also diffuses the elemental melting point depressants into the braze alloy matrix. These actions result in a stronger repair that is less susceptible to incipient melting when the part is returned to service.
Unfortunately, the diffusion braze alloys of the prior art have failed to attain the crucial part-like mechanical and environmental properties demanded by the increased stresses to which today's superalloy articles are subjected. The main reason for this failure is that prior high temperature braze alloys and alloy powder mixtures tend to use only those elements present in the superalloy article being repaired.
This lack of flexibility in the compositions of the prior art has caused a stagnation in the development of truly new braze alloy compositions which employ elements and elemental combinations without regard to the composition of the superalloy substrate. As well, previous multi-constituent alloy compositions were so precisely matched to the particular superalloy to be repaired that it was considered unthinkable to select base metal powders for the mixture based solely on their mechanical and environmental properties.
A need therefore exists for a flexible diffusion braze alloy system capable of accommodating various new elements and base metal powders to increase the strength, flow characteristics, and oxidation resistance of the braze alloy system. A need also exists for a long-term diffusion heat treatment cycle capable of breaking down brittle phases and allowing the elemental melting point depressants to diffuse both into the superalloy substrate and the base metal matrix. A further need exists for a diffusion braze alloy composition which does not rely upon intentional carbon additions for strength. Additionally, a need exists for a diffusion braze alloy composition capable of imparting improved environmental resistance to the superalloy substrate and/or any environmental coating which may be applied to the substrate.
Such a new diffusion braze alloy system desirably employs the elements rhenium, platinum, palladium and/or aluminum in order to improve significantly over the hot corrosion and oxidation resistance properties provided by prior art braze alloys. Additionally, such an improved braze alloy composition preferably uses boron and silicon concurrently as melting point depressants in order to reduce the undesirable mechanical and environmental properties associated with the use of either boron or silicon alone. The present invention addresses these needs.